This invention relates to an idling speed control method for internal combustion engines, and more particularly to a method of the type described which is adapted to prevent engine stalling even if the engine is subjected to a maximum load that can be estimated when an abnormality occurs in a detecting system for detecting the rotational angle position of the engine, which includes a sensor for sensing predetermined crank angle positions of the engine.
In an internal combustion engine, the engine can easily stall due to a drop in the engine speed when the engine is operated in an idling condition at a low temperature of the engine cooling water, or when the engine is heavily loaded during idling operation with electrical loads by head lamps, mechanical loads by air conditioner, etc. in a vehicle equipped with the engine. To eliminate such disadvantage, an idling speed feedback control method has been known e.g. by Japanese Provisional Patent Publication (Kokai) No. 55-98628, which comprises setting a desired idling speed value in dependence upon loads on the engine, detecting the difference between actual engine speed and the desired idling speed, and supplying supplementary air to the engine in a quantity corresponding to the detected difference so as to minimize the same difference, to thereby control the engine speed to the desired idling speed.
As a method of controlling supplementary air quantity control means for supplying the engine with the aforementioned supplementary air is known an idling speed feedback control method disclosed in, e.g., Japanese Provisional Patent Publication (Kokai) No. 58-122350. The disclosed method comprises determining the ratio of operating duty of the supplementary air supply control means to an interval of time at which pulses of a signal indicative of predetermined crank angle positions of the engine are generated, e.g., a top-dead-center (TDC) signal, in dependence upon the difference between actual engine speed and desired idling speed, and driving the supplementary air quantity control means in dependence upon the operating duty ratio, whereby the engine is supplied with a required quantity of supplementary air.
With the foregoing control method, if an abnormality develops in the detecting system which includes a TDC sensor for generating the TDC signal, the wiring of the sensor, and an input device that receives the output signal of the sensor, the TDC signal which gives the TDC interval to serve as a reference no longer arrives as an input. This makes it difficult to properly control the duty ratio of the supplementary air supply control means. When such is the case, it may be considered to use a cylinder-discriminating signal in place of the faulty TDC signal, for control of the duty ratio of the supplementary air supply control means, with the time interval at which pulses of the cylinder-discriminating signal are generated serving as a reference. With such an expedient, however, difficulties arise for the following reason. Specifically, assuming that the engine is, e.g., a four-cylinder engine, the cylinder-discriminating signal pulse is generated once per four TDC signal pulses. If the supplementary air supply control means is operated with the cylinder-discriminating signal pulse interval as a reference when the amount of supplementary air needed corresponds to a duty ratio of 25% to the TDC signal pulse interval as the reference, then a quantity of supplementary air corresponding to the duty ratio of 25% will be supplied to the engine at one time upon the lapse of a predetermined period of time after the cylinder-discriminating signal pulse has been generated. Therefore, though the quantity of supplementary air supplied for all four cylinders is the same regardless of whether the TDC signal pulse interval or the cylinder-discriminating signal pulse interval is used as the reference, the situation is such that when the supplementary air supplied to each of the cylinders is injected with the cylinder-discriminating signal pulse as the reference, the quantity of supplementary air supplied to one or two cylinders is equivalent to that for four cylinders, so that the remaining cylinders receive almost no supplementary air at all, resulting in an imbalance in the quantity of supplementary air supplied to the cylinders. This imbalance in the quantity of supplementary air supplied to the engine cylinders has a major influence upon the engine operating condition and, in certain cases, can lead to problems such as hunting, engine stalling, etc.